1. Field of the Invention
The present invention relates to a key telephone system and, more specifically, to such a system in which both a line in an integrated service digital network (hereinafter abbreviated to "ISDN line") and a public switched telephone network (hereinafter abbreviated to "PSTN line"), commonly used, are accommodated as external lines in the main equipment of the system, and in which extension telephone sets connected to at least two extension lines are each able to call out another terminal over an external line which may be either the ISDN line or the subscriber's telephone line.
2. Background Art
A key telephone system in which both an ISDN line and a PSTN line are accommodated as external lines has hitherto been such that, when the user operates an external line key of an extension telephone set and then calls out another terminal over an external line, the caller is not informed of whether the ISDN line or the PSTN line has been acquired by the operation of the main equipment of the system.
With the conventional system, therefore, a problem arises in which the caller cannot make a quick judgment as to whether the external line acquired is the ISDN line or the PSTN line. This is disadvantageous in that the caller cannot be sure whether various additional services provided in the ISDN are available, even when the ISDN line has actually been acquired by the operation of the main equipment. This means that the caller loses the opportunity of fully utilizing various functions available in the ISDN.
For instance, if the external line acquired is an ISDN line, the party who has received the call (i.e., the calling-in party) is informed of the number of the calling-out party and, accordingly, is aware who is calling before their conversation starts. However, the calling-out party, who cannot be aware of this fact, may start a conversation by introducing himself. Another drawback is that, although modemless data transmission is possible with an ISDN line, this function may not be utilized by a calling-out party.
Conversely, if a calling-out party who is actually using a PSTN line erroneously believes that he is using an ISDN line, he may encounter the following trouble. When the calling-out party starts a conversation with the calling-in party whom he considers, on the ground of his erroneous belief, to be already aware of who he is, the conversation becomes incoherent.
Another problem with the conventional system is that an ISDN line and a PSTN line employ different calling-out methods. In order to enable the same calling-out operation to be performed whether an ISDN line or a PSTN line is used by a calling-out party, complicated control is necessary while high cost is incurred.